A high-amplitude 2T mode of vortex-induced vibration for a light body in XY motion

Although there are a great many papers dedicated to the problem of a cylinder vibrating transverse to a fluid flow (Y-motion), there are almost no papers studying the more practical case of vortex-induced vibration in two degrees of freedom (XY-motion) where the mass and natural frequencies are precisely the same in both X and Y directions. We have designed the present pendulum apparatus to achieve both of these criteria. Even down to the low mass ratios, where m* = 6, it is remarkable that the freedom to oscillate in-line with the flow, affects the transverse vibration surprisingly little. There is, however, a dramatic change in the fluid-structure interactions when mass ratios are reduced below m* = 6. A new amplitude response branch with significant streamwise motion appears, in what we call the super-upper branch, yielding massive amplitudes of 3 diameters peak-to-peak (A(Y)* similar to 1.5). We discover a corresponding periodic vortex wake mode, comprising a triplet of vortices being formed in each half cycle, in what we define as a 2T mode. The extensive studies of VIV for Y-only body motions, built up over the last 35 years, remain of strong relevance, for m* > 6. It is only for light bodies, m* < 6, that one observes a rather marked departure from previous results. (C) 2003 Elsevier SAS. All rights reserved.